Carbon materials, which have been used mainly for adsorbents and the like, are being investigated for wide applications because they have basic properties such as physical properties for electronic materials such as conductivity, high thermal conductivity, a low coefficient of thermal expansion, lightness, and heat resistance. In recent years, their physical properties for electronic materials have received attention, and they are thus used in or investigated particularly for electronic material fields such as lithium ion secondary battery negative electrodes and capacitor electrodes.
Such carbon materials are produced by using coconut husks, coal coke, coal or petroleum pitch, furan resins, phenol resins or the like as a raw material and subjecting the raw material to carbonization.
In recent years, there have been attempts to develop carbon materials by allowing such carbon materials to contain other elements to further extend the range of the physical properties of the carbon materials. Under these circumstances, nitrogen-containing carbon materials have recently received attention because of reports that they provide improved electrochemical characteristics in applications such as lithium ion secondary battery negative electrodes and capacitor electrodes and electrodes for electrolysis (refer, for example, to Patent Documents 1 and 2) and reports on the characteristics thereof as adsorbents (refer, for example, to Patent Document 3) and as hydrogen-occluding materials (refer, for example, to Patent Documents 4 and 5).
Conventionally known methods of producing nitrogen-containing carbon materials and such nitrogen-containing carbon materials are listed below.
As the methods of producing nitrogen-containing carbon materials, there are mainly known (1) a method of subjecting a low molecular nitrogen-containing organic compound as a raw material to chemical vapor deposition (CVD); (2) a method of polymerizing a low molecular nitrogen-containing organic compound as a raw material and then subjecting the resulting resin to carbonization; and the like.
In the above-described (1), there are known, for example, a method of depositing a nitrogen-containing organic compound such as pyrrole on a substrate (refer to Patent Documents 6 and 7); a method of depositing a nitrogen-containing organic compound in the pores of a porous body (refer to Patent Document 3); a method of depositing an acyclic organic compound such as acetonitrile on a carbon material (refer to Patent Documents 8 and 5); a method of polymerizing 2,3,6,7-tetracyano-1,4,5,8-tetraazanaphthalene and carbonizing the resulting polymer at a high temperature (refer to Patent Document 9); and the like.
In the above-described (2), there are known, for example, a method of carbonizing a melamine resin, a urea resin, an aniline resin at a high temperature (refer to Patent Document 10); a method of carbonizing a polyimide at a high temperature (refer to Patent Document 1); a method of carbonizing a polyaniline at a high temperature (refer to Patent Document 2); a method of carbonizing a polypyrrole at a high temperature (refer to Patent Document 11); a method of mixing phthalocyanine with a precursor of a furan resin and then carbonizing the resulting mixture at a high temperature (refer to Patent Document 12); a method of carbonizing polyacrylonitrile at a high temperature (refer, for example, to Patent Document 13); and the like.
However, neither the method (1) nor the method (2) is an economical production. In addition, they are not a satisfactory method of producing a material having a high nitrogen content and a low hydrogen content.
That is, the method (1) is not preferred because a process using CVD itself is not suitable for industrial mass-production, and because when a halogen-containing compound such as chlorine is used in a CVD process there is a problem of corrosion of materials.
In the method (2), an expensive resin such as a resin whose monomer used as a raw material is produced by a complicated production process such as multistage reaction or a resin which is not industrially mass-produced is used as a raw material to produce a carbonized product. Therefore, considering processes starting from a basic raw material, there is a problem of consuming an enormous amount of raw materials and energy until a carbon material is produced. Further, since a polymerization process and a process of forming resins or fibers are complicated, a nitrogen-containing carbon material obtained by carbonization consumes an increasingly greater amount of raw materials and energy. This increases the cost of nitrogen-containing carbon materials, making it unsatisfactory for them to be supplied to various applications. Furthermore, there have been problems such as a problem that the yield of a carbonized product at carbonization is low; a problem that the nitrogen content in the obtained nitrogen-containing carbon material is low; or a problem that when carbonization temperature is lowered or carbonization time is shortened in order to increase the nitrogen content, the progress of carbonization is suppressed to increase hydrogen content, leading to insufficient formation of a conjugated structure, which prevents development of characteristics as a carbon material in the first place. In addition, use of halogen such as chlorine or application of high pressure in the carbonization process is industrially disadvantageous in terms of materials and operation.
Characteristics as a substance of nitrogen-containing carbon materials having been produced by the above-described methods will be summarized below as follows.
A nitrogen-containing carbon material obtained by CVD generally has a low nitrogen content and a high hydrogen content, contains a residual nitrile group or a residual halogen group, and tends to have a shorter interlayer spacing of a layered structure. For example, in Patent Documents 6, 9, and 5, a nitrogen-containing carbon material is produced by CVD using pyrrole, 2,3,6,7-tetracyano-1,4,5,8-tetraazanaphthalene, acetonitrile, or cyanogen bromide as a raw material. However, it has a low nitrogen content or a high hydrogen content, contains a residual nitrile group or a residual halogen group, or has a short interlayer spacing. That is, in the X-ray diffraction pattern thereof, the peak location of the angle of diffraction (20) corresponding to the (002) plane is 26.50 (corresponding to a interlayer spacing of 3.36 angstroms).
Patent Document 11 discloses an example of producing a nitrogen-containing carbon material obtained by a method in which pyrrole is polymerized and the resulting polymer is carbonized at a high temperature. In the X-ray diffraction pattern of the resulting nitrogen-containing carbon material, the peak location of the angle of diffraction (2θ) corresponding to the (002) plane is 26.0°, which corresponds to a interlayer spacing of 3.42 angstroms. Patent Document 11 also discloses the laser Raman spectrum of the nitrogen-containing carbon material, wherein the peaks are located at 1,600 cm−1 corresponding to crystallinity and 1,350 cm−1 corresponding to amorphousness, showing substantially no peak shift. The 1,600 cm−1 and 1,350 cm−1 peaks are clearly separated, indicating that the Raman peaks each have a narrow half-width.
The above-described Patent Document 4 discloses a nitrogen-containing carbon material produced by CVD from N-vinyl-2-pyrrolidone. The laser Raman spectrum data of the resulting nitrogen-containing carbon material shows that the nitrogen-containing carbon material has a peak at 1,350 cm−1 with a sharp half-width of 97 cm−1. Patent Document 4 also discloses an example in which a nitrogen-containing carbon material is produced by pulverizing graphite powder at a very high acceleration of pulverization under a high-pressure nitrogen atmosphere. However, the 1,350 cm−1 peak has a relatively sharp half-width of 87 cm−1 at the most.
The half-width or the broadness of a peak of the laser Raman spectrum is an index showing crystallinity, and it is well known that the peak becomes sharper as the crystallinity increases (for example, Non-Patent Document 1).
That is, the nitrogen-containing carbon materials obtained by conventional techniques are those having at least one of the following four characteristics: (i) having a low nitrogen content, having a high hydrogen content, and/or containing a nitrile group and a halogen group; (ii) having a short interlayer spacing of the (002) plane as measured by X-ray diffraction; (iii) having a peak of the spectrum measured by laser Raman spectroscopy which is not shifted; and (iv) having a peak with a small half-width, showing high crystallinity.
It is known that carbon materials having a higher nitrogen atom content are more advantageous as a carbon material used in electronic material applications such as lithium ion secondary batteries and capacitors (for example, Patent Documents 7 and 14). In addition, a lower hydrogen content is more advantageous to electronic physical properties such as electron conductivity because the lower the hydrogen content, the more the conjugated structure develops. Preferably, a functional group is not present.
It is known that a carbon material having a low crystallinity is known as a non-graphitizable carbon (also referred to as a hard carbon), which is advantageous to the improvement of capacitance (for example, Non-Patent Document 2). It is also known that the low crystallinity makes it possible to use propylene carbonate which is excellent in low temperature operating characteristics as a solvent for an electrolyte (for example, Non-Patent Document 3).
Moreover, a large interlayer spacing is advantageous to the formation of an intercalation compound. In the case of a lithium ion secondary battery, it is advantageous to the insertion of lithium ions into and elimination from between the layers.
Therefore, a new carbon material having none of the above characteristics (i) to (iv) has been desired.
Patent Document 1: JP-A-2001-80914
Patent Document 2: JP-A-10-21918
Patent Document 3: JP-A-2004-168587
Patent Document 4: JP-A-2005-798
Patent Document 5: JP-A-2003-277026
Patent Document 6: JP-A-7-90588
Patent Document 7: JP-A-9-27317
Patent Document 8: JP-A-2004-342463
Patent Document 9: JP-A-2003-137524
Patent Document 10: JP-A-2000-1306
Patent Document 11: JP-A-8-165111
Patent Document 12: JP-A-2004-362802
Patent Document 13: JP-A-8-180866
Patent Document 14: JP-A-2005-239456
Non-Patent Document 1: Large-capacity secondary battery for automobiles, p. 140, CMC Publishing (2003)
Non-Patent Document 2: Carbon material for negative electrode lithium ion secondary battery, P. 4, Realize Co. (1996)
Non-Patent Document 3: Carbon material for negative electrode for lithium ion secondary battery, P. 11, Realize Co. (1996)
In view of the above circumstances, it is an object of the present invention to provide an energy-saving, resource-saving method of producing a nitrogen-containing carbon material characterized by utilization of a monomer directly derived from a basic chemical raw material such as natural gas or a fraction from a naphtha cracker or produced as a by-product thereof, simplicity of the polymerization step and the polymer pulverization step which is a step after polymerization, and a high carbonized-product yield in the step of carbonizing the obtained pulverized polymer.
It is another object of the present invention to provide a method of producing a nitrogen-containing carbon material which is industrially simple and allows mass production.
It is a further object of the present invention to provide a new nitrogen-containing carbon material having a high nitrogen atom content, a low hydrogen atom content, a low residual ratio of a nitrile group and a halogen group, a layered structure, a long interlayer spacing, and a specific peak shift in the laser Raman spectrum.